This invention relates to antenna systems for the reception of electromagnetic waves, and more particularly, to compact and effective antenna systems capable of efficiently intercepting VHF or UHF signals of the frequencies used in television and FM radio broadcasting, and transferring them to the input of a signal conversion device. In general, the present invention relates to antenna systems in which the receptor appears to be electrostatically rather than inductively responsive, and is coupled to a low impedance, resistively terminated transmission line.
Numerous antenna configurations have heretofore been proposed, consisting in general of dipoles, loops, or long-wire type devices, and variations or combinations of them. For VHF and UHF reception, various forms of dipoles are today used almost exclusively; loops are also used for VHF as well as often used for direction finders, or in combination with ferrite rods for AM broadcast reception.
Electrostatic antennas, using solid flat plates used for reception of electromagnetic waves, are effective only in that part of the electromagnetic spectrum where the capacity reactance of the solid plate matches the transmission line. These devices have no directional properties and must be dimensioned to the frequency to be received. These devices also exhibit very little radiation when connected as transmitting antennas, and only in this respect, do they resemble this invention. This invention defies the generally accepted theory of reciprocity in antenna systems which says that an antenna must be capable of transmitting as well as receiving to be a good receiving antenna, but tests bear out the fact that the subject of this invention provides reception equal to or superior to dipoles properly tuned and oriented, and does not radiate any appreciable signal when driven as a transmitting antenna.
As is well-known, transmitting antennas radiate a combined field of electric and magnetic energy, and the interchange of energy in the two fields results in a composite field of energy commonly identified as an electromagnetic wave in the far zone (i.e., several wavelengths away from the transmitting antenna). This wave may be visualized as a spherical or isotropic field when radiated in free space from an antenna ideally coupled to the characteristic impedance (120 pi) of free space. At distances comparable to those of a typical TV receiver, the electromagnetic wave front is only a small area segment of the outer boundaries of the isotropic field of energy. Thus, a signal appears to a remote antenna as a plane wave whose electric and magnetic fields are 90.degree. apart and perpendicular in the direction of travel of the wave front.
The design of the dipole and lopp families of antenna is predicated on these known characteristics of electromagnetic wave propagation. Dipoles utilize the energy of both the electric and magnetic fields, so that currents are induced in the antenna elements, and voltage gradients are established as functions of the dimensions of the antenna with respect to the wavelengths of the incident signals.
A dipole is characteristically a basically resonant narrow-band device, with a marked bi-directional pattern. For optimum efficiency, therefore, it must be tuned and accurately directed. Typically, as a result of their electromagnetic properties, receiving dipoles also exhibit substantial reradiation of the incident field with attendant energy loss to their surroundings. Means such as parasitic elements, reflectors and directors are often used with broad-band folded dipoles to provide, to the extent feasible, multiple modes of resonance to cover the desired frequency spectrum, and to recapture reradiated energy resulting from current flow in the antenna elements. The extent of reradiation is a measure of the inefficiency of known dipoles.
In contrast to dipoles, loop antennas are essentially magnetic field receiving devices, the sensitivity of which is a function of area and the number of turns. They must of necessity by physically larger than antennas in accordance with the present invention. Moreover, like dipoles, but unlike antennas in accordance with the present invention, loop antennas suffer significant losses due to reradiation, because they are closed circuits in which current flow is sought to be maximized.
Among the numerous known prior art patents directed to antenna configurations or systems are: U.S. Pat. Nos. 2,039,988, issued May 5, 1936, to Graves, Jr.; 2,166,750, issued July 18, 1939, to Carter; 2,648,001, issued Aug. 4, 1953, to Rowland; 2,761,140, issued Aug. 28, 1956, to Ashton; 2,821,710 issued Jan. 28, 1958, to Hale; 2,990,447 issued June 27, 1961, to McDougal; 3,167,775, issued Jan. 26, 1965, to Guertler; 3,231,894, issued Jan. 25, 1966, to Nagai; 3,344,425, issued Sept. 26, 1967, to Webb; 3,454,951, issued July 8, 1969, to Patterson, et al.; 3,689,929, issued Sept. 5, 1972, to Moody; and 3,716,861, issued Feb. 13, 1973, to Root; and German Pat. No. 1,019,717, issued Nov. 21, 1957, to Kathrein.
Also known are U.S. Pat. Nos. 1,606,775, issued Nov. 16, 1926, to Nyman; 1,875,951, issued Sept. 6, 1932, to Taylor et al; 2,135,037 issued Nov. 1, 1938, to Landon; 2,189,309, issued Feb. 6, 1940, to Carlson et al; 2,218,083, issued Oct. 15, 1940, to Carlson et al; 2,558,339 issued June 25, 1951, to Cohen; 3,013,268 issued Dec. 12, 1961, to Hamel et al; 3,079,602, issued Feb. 26, 1963, to Hamel et al; 3,210,768, issued Oct. 5, 1965, to Hudock, et al; 3,373,533, issued Mar. 12, 1968, to Blaisdell; 3,530,473 issued Sept. 22, 1970, to Ives; 3,820,117, issued June 25, 1974, to Hall et al; 3,971,032, issued July 20, 1976, to Munson et al; 3,984,834 issued Oct. 5, 1976, to Kaloi; 4,040,060, issued Aug. 2, 1977, to Kaloi.
Each of the foregoing patents, among numerous others, relates to a proposed antenna with points of superficial similarity to aspects of the applicant's antenna system, but none discloses the applicant's antenna structure or appears capable of realizing the operative advantages of the present system. For example, although it is suggested in U.S. Pat. No. 3,716,861 that in a loop antenna (which the applicant's is not) "a serptentine configuration" may increase capacitive reactance and radiation resistance, no suggestion is to be found for a configuration like that of the present invention in which reradiation and its attendant losses are minimized. So, too, in the matter of directionality, which, according to the disclosure of that patent is to be controlled by the disposition of the undulations or by the size of the undulations.
The present invention has, therefore, as its principal object, the provision of a compact high efficiency broadband antenna system, the characteristics of which are substantially omni-directional, but which, as will be shown, has a sharp null zone, which may be used to reduce undesired interference levels.
The foregoing and other objects are realized, in a presently preferred form of the invention, by a system which employs a novel complement of a multi-resonant substantially non-reradiating receptor and output matching load coupler and a low VSWR transmission line and signal conversion load coupler.
In a presently preferred form of the invention, the receptor has a plurality of individual segments disposed in a symmetrical array, the segments being wire-like conductive members, sinuous in configuration. The conductors defining the segments and their elements are of small cross-sectional dimension, it having been found that, in general, the smaller the cross-section of the conductors and the more closely they are spaced the more satisfactory the performance of the antenna. Although, consistent with the principles of the invention, the conductors may be wire, or preferably, created by printed circuit techniques, they are sometimes referred herein as "wire-like" to signify their small cross-sectional area, and their tight-folded configuration so as to distinguish them from large self-supporting antenna-forming elements such as tubing or castings.
The electrically symmetrical receptor apparently presents to the sending end of the transmission line via the coupler system a broad-band frequency response and impedance comparable to that of free space, i,e., 120 pi. The sending end of the transmission line in the presently preferred embodiment consists of a series loading coil which is connected to the receptor at the electrical center point of the receptor, and is both electrically and inductively coupled to the low impedance low VSMR transmission line.
Since the present receptor is electrically symmetrical with respect to its feed point, it is relatively insensitive to the magnetic component of the electromagnetic field. Under these conditions a very small RF surface current flows on the receptor, and consequently its low ohmic resistance does not result in a significant reradiation of the field received. Any currents which result from the absorption of the electric field will appear at the common junction point of the two halves of the receptor, in phase relationships which vary with frequency, due to the multi-resonant modes of the interconnecting monopoles, thereby reducing the magnitude of the current flow in the receptor elements and raising its radiation resistance to a point where, it is believed, the receptor becomes essentially a bridging source of energy between the transmission line coupling system and free space.